This invention relates to a method and apparatus for handling the off-gases from a vessel in which impure iron is converted into steel.
A known method of making steel involves use of a refractory lined vessel which is supported in a ring from which horizontal trunnion pins extend. The trunnion pins are supported in journals which enable the vessel to be tilted clear of the primary gas collection hood so that the vessel may be charged with hot metal and scrap and so its contents can be poured out from the mouth.
When a bottom blown converter vessel is used, finely divided solid reactants and process gases are injected into the molten metal within the vessel through multipassageway tuyeres which extend through the refractory lining and open into the vessel beneath the surface of the molten metal. In one stage of the refining process, oxygen in which finely divided burnt lime is suspended is injected into the molten metal through one passageway of the submerged tuyeres. A hydrocarbon fluid is injected simultaneously through the other passageway. The reaction between carbon in the impure molten metal and oxygen causes very high temperatures to be developed in the vicinity of the tuyeres. This high temperature would ordinarily result in damage to the tuyeres and the refractory bottom of the vessel but the hydrocarbon fluid effects cooling and thereby preserves the tuyeres and vessel bottom.
The carbon and oxygen reaction results in evolution of carbon monoxide which is an atmospheric pollutant and is combustible or explosive. The evolved or off-gases contain suspended fine particulates which must be removed from the off-gases before the gas is collected for use as a fuel or before it is burned to carbon dioxide and discharged to the atmosphere.
When the vessel is tilted fumes and noxious gases can escape to the atmosphere within the steel plant. This effect is mitigated by a secondary hood which collects the off-gases when the vessel mouth is tilted away from the primary hood. The secondary hood is connected by means of a duct into the gas cleaning system. It has been customary to interpose a damper in the duct between the secondary hood and the gas cleaning system and this damper is closed except when the vessel is tilted or turned down in which case the damper is opened so that the gas cleaning system draws escaping off-gases through the secondary hood. It is important that the damper, valve or whatever means is used to shut off the secondary hood be leak-proof lest oxygen from the air be drawn into the secondary hood to mix with carbon monoxide which evolves from the vessel and is collected by that part of the system which originates with the primary hood. Admission of oxygen containing air in the hot carbon monoxide stream, of course, presents an explosion hazard to the gas cleaning system.